Fix noise_model initialized in direct_marl_env

[NoneJou072]

Description

This change fixes bugs in DirectMARLEnv class, where previously _action_noise_model and _observation_noise_model cannot successfully initialized when the noise-model-cfg passed in.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there